Re: [PATCH] nfs: block notification on fs with its own ->lock

[Vasily Averin <vvs@virtuozzo.com>]

On 16.12.2021 20:20, J. Bruce Fields wrote:
> From: "J. Bruce Fields" <bfields@redhat.com>
> 
> NFSv4.1 supports an optional lock notification feature which notifies
> the client when a lock comes available.  (Normally NFSv4 clients just
> poll for locks if necessary.)  To make that work, we need to request a
> blocking lock from the filesystem.
> 
> We turned that off for NFS in f657f8eef3ff "nfs: don't atempt blocking
> locks on nfs reexports" because it actually blocks the nfsd thread while
> waiting for the lock.
> 
> Thanks to Vasily Averin for pointing out that NFS isn't the only
> filesystem with that problem.
> 
> Any filesystem that leaves ->lock NULL will use posix_lock_file(), which
> does the right thing.  Simplest is just to assume that any filesystem
> that defines its own ->lock is not safe to request a blocking lock from.
> 
> So, this patch mostly reverts f657f8eef3ff and b840be2f00c0, and instead
> uses a check of ->lock (Vasily's suggestion) to decide whether to
> support blocking lock notifications on a given filesystem.  Also add a
> little documentation.
> 
> Perhaps someday we could add back an export flag later to allow
> filesystems with "good" ->lock methods to support blocking lock
> notifications.
> 
> Reported-by: Vasily Averin <vvs@virtuozzo.com>
> Signed-off-by: J. Bruce Fields <bfields@redhat.com>

Reviewed-by: Vasily  Averin <vvs@virtuozzo.com>